Ultrasonic flow measuring devices are applied often in process- and automation technology. They permit simple determination of volume flow and/or mass flow in a pipeline.
Known ultrasonic flow measuring devices frequently work according to the travel-time difference principle. According to this principle, the different travel times of ultrasonic waves, especially ultrasonic pulses, so-called bursts, are evaluated relative to the flow direction of the fluid. For this, ultrasonic pulses are sent at a certain angle to the tube axis both with the flow as well as also counter thereto. From the travel-time difference, the flow velocity and therewith, in the case of known diameter of the pipeline section, the volume flow can be determined.
The ultrasonic waves are produced, respectively received, with the aid of so-called ultrasonic transducers. For this, ultrasonic transducers are mounted in the tube wall of the relevant pipeline section. The ultrasonic transducers are, normally, composed of an electromechanical transducer element, e.g. a piezoelectric element, and an ultrasonic window. The ultrasonic waves are produced as acoustic signals in the electromechanical transducer element and led to the ultrasonic window and from there in-coupled into the fluid, or measured medium. The ultrasonic window is also referred to as a membrane.
Between the piezoelectric element and the ultrasonic window, a so called adapting, or matching, layer can be arranged. The adapting, or matching, layer performs, in such case, the function of transmission of the ultrasonic signal and simultaneously the reduction of a reflection on interfaces between two materials caused by differing acoustic impedances.